The present invention relates to a novel propylene polymer composition useful for production of moldings.
Crystalline polypropylene is a useful resin as a general-purpose resin because of its high stiffness, hardness, tensile strength, and heat resistance.
Crystalline polypropylene, however, is not sufficiently high in impact resistance and has a diadvantage of being poor in physical properties at low temperatures.
In general, stiffness, hardness and heat resistance of plastic material are in an antinomic relation with impact resistance thereof, and it is quite difficult to design so that all the stiffness, hardness, heat resistance and impact resistance are sufficiently satisfactory.
In order to extend applications of propylene polymers, however, it is necessary to improve both impact resistance and stiffness.
Such improved propylene polymers can be used in place of an ABS resin well balanced in impact resistance and stiffness.
Under such circumstances, to improve impact resistance of propylene polymers, several proposals have been made. For example, a method in which an elastomer exemplified by an ethylene-propylene rubber (EPR) is compounded to polypropylene has been proposed.
In this method, however, the elastomer is dispersed insufficiently as compared with a composition which is produced by two-stage polymerization and compounded in the second stage reactor, and thus can provide only moldings which are decreased in weld strength, luster and further in stiffness. Furthermore, compounding of EPR increases the cost of a polypropylene composition.
A method of preparing propylene-ethylene random copolymers by copolymerizing propylene with other .alpha.-olefin, e.g., ethylene has been known for the same purpose above.
Further, a method of preparing a propylene polymer composition comprising two or more components obtained by multi-stage polymerization has been known for the same purpose as above.
Random copolymers obtained by the conventional methods, however, are improved insufficiently in low temperature impact resistance as compared with homopolypropylene. If the ethylene content is increased, stiffness, strength, heat resistance, etc. are abruptly decreased.
In conventional copolymer compositions obtained by the multi-stage polymerization method, the low temperature impact resistance value is greatly increased as compared with polypropylene. However, stiffness, hardness and heat resistance are decreased.
A number of proposals have been made to overcome the above problems of the multi-stage copolymerization method. For example, Japanese Patent Application Laid-Open Nos. 115296/1975 and 4588/1977 disclose a method in which block copolymerization of ethylene and propylene is carried out at multi-stages. Japanese Patent Publication Nos. 8207/1972, 13231/1974, and 13514/1974 disclose an improved method in which a third component is added to the catalyst. Japanese Patent Application Laid-Open Nos. 764/1980, 152095/1979, and Japanese Patent Publication No. 8011/1980 disclose an improved method in which a specified catalyst is used.
The above methods are intended to minimize the reduction of stiffness as compared with a polypropylene homopolymer. However, the value of stiffness equal to or more than that of the homopolymer has not been obtained.
An object of the present invention is to provide a propylene copolymer composition which is suitable for molding of high impact resistant and high stiffness moldings.
In view of the above circumstances of conventional techniques, the present inventors made investigations to develop a propylene polymer composition which permits production of moldings having high impact resistance and high stiffness without addition of specific additives. As a result, it has been found that high stiffness moldings can be obtained only by using a propylene polymer composition containing a specified propylene homopolymer component and a specified ethylene-propylene random copolymer as described hereinafter in a specified ratio. Based on the findings, the present invention has been accomplished.
The present invention relates to a propylene polymer composition comprising:
60 to 94% by weight of a propylene homopolymer having: PA0 6 to 40% by weight of a propylene-ethylene random copolymer having:
(a) an intrinsic viscosity, [.eta..sub.1 ], of 0.5 to 5 dl/g; PA1 (b) a xylene soluble content at the ordinary temperature, Xs, of not more than 3% by weight, and a difference (.DELTA.[.eta..sub.1 ]) between an intrinsic viscosity ([.eta..sub.1 ]) of a xylene insoluble portion (at the ordinary temperature) and an intrinsic viscosity ([.eta..sub.1 ]) of a xylene soluble portion, i.e., .DELTA.[.eta..sub.1 ]=intrinsic viscosity ([.eta..sub.1 ]) of insoluble portion--intrinsic viscosity ([.eta..sub.1 ]) of soluble portion of not more than 0.5 dl/g; PA1 (c) an isotactic pentad fraction, IP, falling within the range represented by the equation: EQU IP.gtoreq.-0.624[.eta..sub.1 ]+97.5; PA1 (d) a crystallization temperature (Tc) falling within the range represented by the following equation: EQU Tc.gtoreq.-2.33[.eta..sub.1 ]+116.0; PA1 (e) an intrinsic viscosity, [.eta..sub.2 ], of not less than 2.8 dl/g; PA1 (f) an ethylene content of 30 to 80% by weight. PA1 (a) The intrinsic viscosity, [.eta..sub.1 ], of the propylene homopolymer is 0.5 to 5 dl/g, preferably 1 to 4 dl/g, and more preferably 1.1 to 3.0 dl/g. If the intrinsic viscosity is more than 5 dl/g, fluidity at the time of melting is low. On the other hand, if it is less than 0.5 dl/g, the strength of the molding is decreased. PA1 (b) The xylene soluble content at the ordinary temperature (Xs) of the propylene homopolymer is not more than 3% by weight, preferably 2 to 0.1% by weight, and more preferably 1.5 to 0.2% by weight. PA1 (c) The isotactic pentad fraction (IP) of the propylene homopolymer of the present invention is in the range represented by the equation: EQU IP.gtoreq.-0.624[.eta..sub.1 ]+97.5, PA1 (d) The crystallization temperature Tc of the propylene homopolymer is within the range represented by the equation: EQU Tc.gtoreq.-2.33[.eta..sub.1 ]+116.0,
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